SELF FERTILIZATION 317 



at the time of the reduction division ; but in the male the pollen-tube corresponds 

 not to a diploid somatic tissue, but to a gamete in which there has occurred a 

 reduction division, causing a segregation, and some pollen-tubes possess, 

 therefore, S 1? while others possess S 2 . In other plants S 3 and S 4 , or S x and 

 S 3 , may substitute for S x and S 2 . Now, fertilization is possible if the substance 

 which characterizes the pollen-tube is not present in the female style, other- 

 wise an inhibition in the downgrowth of the pollen-tube takes place and 

 fertilization is prevented ; in other words, fertilization cannot follow if the 

 stimulating effect on the pollen-tube, which is due to a difference in the sub- 

 stances characterizing pollen-tube and style, is lacking. 



The fertilization experiments which East carried out yielded results which 

 were in agreement with his assumption. There are involved in this case, 

 conditions in which the response of certain tissues to each other depends upon 

 the presence of the same, or of two different substances, in the interacting 

 cells and tissues ; when these substances are identical the outcome of the re- 

 action is unfavorable. 



If a very large number of various kinds of homoiogenous fertilization ex- 

 periments are carried out, many different factors can be made to interact with 

 each other and a variety of combinations occurs. It is thus found that if two 

 individuals which differ in a factor are crossed, the F 1 hybrids are fertile with 

 each of the parents. As far as the behavior of the F x hybrids towards one 

 another is concerned, they can be placed in four groups, the members of each 

 group being sterile with the other individuals of the same group, and fertile 

 with all members of the other groups. 



However, in some species of Nicotiana self-fertilization leads to fertile 

 progeny; this is so in Nicotiana Langsdorfh. Here, possession of the same 

 factors by style and pollen-tube does not interfere with the rapidity of down- 

 growth, while in Nicotiana alata it leads to sterility. 



As to the mechanisms underlying self -sterility, in Medicago sativa, after 

 self-pollination, not only does the pollen-tube grow into the ovary more slowly, 

 but also the number of eggs which are fertilized is smaller; in a number of 

 instances the pollen-tube does not enter the micropyle of the ovulum, and if 

 fertilization should take place, abortion occurs rather frequently. More recent 

 investigations indicate that in certain instances, as presumably also in some 

 other plants, self-sterility is genetically determined and depends upon the 

 presence in pollen and ovule of two recessive genes, which must be present 

 in double dose in order to insure self-sterility. These genetic factors determine 

 the mechanism which causes a very slow rate of downgrowth of the pollen- 

 tube into the style; in the latter, it may also induce the formation of a separat- 

 ing wall, preventing the further movement of the pollen-tube towards the 

 ovule. According to Yasuda, an ovarian secretion diffuses into the style and 

 has this inhibiting effect on the pollen-tube ; in addition, this substance may 

 inhibit the germination of the pollen. The ovarian product may also exert 

 an inhibition on otherwise non-self -sterile pollen. According to Eysh, spray- 

 ing of the flowers of self-sterile plants with alpha naphthalene acetamide di- 

 rectly before or after self-pollination neutralizes this ovarian secretion and 



